Flask in rosnode can't be terminated by control + C

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I try to run flask code in ros like this. Every thing seems good until I try to terminate it by press Contrl + C. But it didn't work... Is there anyone know what's the problem or anyway I can terminate it?

if __name__ == '__main__':
    rospy.init_node('client')
    parser = ROSArgparse(relative='client/')
    args = parser.parse_args()
    client = InteractionClient(args)
    app = Flask(__name__)

    @App.route('/get_image/<int:idx>')
    def image(idx):
        ###The view function for sending images with id
        arr = load_image(idx)
        img = Image.fromarray((arr*255).astype('uint8'))
        # create file-object in memory
        file_object = io.BytesIO()
        # write PNG in file-object
        img.save(file_object, 'PNG')
        # move to beginning of file so `send_file()` it will read from start
        file_object.seek(0)
        return send_file(file_object, mimetype='image/PNG')

    app.run(debug=True)